1. Field of the Invention
The present invention relates to a process for preparing a highly homogeneous cellulose solution which can be used in the production of cellulosic fibers or films. More particularly, the present invention relates to a process for preparing a highly homogeneous cellulose solution by directly dissolving cellulose powder in a liquid N-methylmorpholine-N-oxide solvent in an extruder.
2. Description of the Prior Art
Graenacher et al. first suggested a process for preparing a cellulose solution in a tertiary amine oxide in U.S. Pat. No. 2,179,181. Subsequently, more efficient and economical processes have been proposed.
U.S. Pat. Nos. 4,142,913, 4,144,080, 4,196,262 and 4,246,221 disclose processes which comprise swelling cellulose in an aqueous solution of a tertiary amine oxide having a water content of 22% or more, which will not dissolve the cellulose due to its relatively high water content, and subsequently distilling off the excess amount of water in the resulting slurry to dissolve the cellulose therein. The processes require an extended period of time to perform the distillation under reduced pressure. However, the distillation tends to cause discoloration of the solution due to the thermal degradation of the solvent and cellulose. As a whole, these processes are complicated as well as time and energy consuming.
Similarly to the above U.S. Patents, European Patent No. 356,419 to Zikeli et al. discloses a continuous process of producing 72 kg of cellulose solution per hour, wherein distillation/concentration is a still essential step. More specifically, the process comprises swelling cellulose in an aqueous N-methylmorpholine-N-oxide (hereinbelow, referred to as "NMMO") solution having a water content of 40%, and distilling the swollen slurry under reduced pressure in a screw extruder with fan-shaped flights. Quigley also uses a thin film evaporator as a reduced-pressure distillater as disclosed in WO 94/06530. However, not only is the highly viscous slurry not efficiently concentrated, but the reduced pressure distillation inevitably requires complicated apparatus. Thus, disadvantages of these processes are their low productivity and complexity.
U.S. Pat. No. 4,211,574 suggests another method of producing a cellulose solution, wherein a cellulose pulp sheet is soaked and swollen at a temperature of 85 to 95.degree. C. in a tertiary amine oxide solution having a water content of 5 to 15%, sufficiently low to dissolve the cellulose therein, and is subjected to stirring and heat treatment without the concentration step. In this method, however, since the amine oxide solvent can only partially swell the surface of the cellulose pulp sheet and consequently will form a coating thereon, the cellulose in the solution tends to remain undissolved because the penetration of the amine oxide solvent is prohibited by the coating. Once coatings are formed on the surface, a homogeneous solution cannot be obtained even by vigorous stirring or heating. Thus, a disadvantage of the process is that it fails to produce a homogeneous cellulose solution.
U.S. Pat. No. 4,416,698 proposes still another process for producing a cellulose solution. In accordance with the process, cellulose powder and a solid NMMO are simply mixed and transferred to an extrusion apparatus wherein the mixture is dissolved to give a cellulose solution. The two powder components, especially when used in a large amount, will not make a sufficiently uniform mixture to leave a part of the cellulose powder undissolved in the resulting solution. In combination with a lower fluidity, this makes the process unsuitable for industrialization and mass production.
The various processes for producing a cellulose solution in a tertiary amine oxide suggested in the prior art patents as mentioned above may be categorized into three groups as follows:
Group I: Processes as disclosed in U.S. Pat. Nos. 4,142,913, 4,144,080, 4,196,262, 4,246,221, 4,290,855 and 4,324,539, and European Pat. No. 356,419. ##STR1##
Group II: Process as disclosed in U.S. Pat. No. 4,211,574. ##STR2##
Group III: Process as disclosed in U.S. Pat. No. 4,416,698. ##STR3##
As set forth above, the prior art processes have the problems of being complicated processes and/or requiring apparatus due mainly to the reduced-pressure distillation, and yielding unsatisfactory homogeneity resulting from a high NMMO concentration in the solution.
As a result of extensive investigation, the present inventors discovered that the problems of the prior art mentioned above can be eliminated by using fibrillar cellulose powder or fluff, instead of a cellulose pulp sheet as conventionally used, in a highly concentrated aqueous solution of NMMO having a water content of 5 to 20% and maintained at the temperature range of from 50 to 130.degree. C. This process can provide a homogeneous solution in a simpler manner, while not forming a coating on the surface of cellulose as in a process using a cellulose pulp sheet.
Contrary to the conventional processes which require swelling in a tertiary amine oxide and subsequent distillation, the process of the present invention can directly produce a cellulose solution without swelling by using fibrillar cellulose powder. Because the solvent can easily penetrate through the fibrillar cellulose, the process of the invention is a simpler process that is able to obtain complete dissolution of cellulose in a shorter period of time.